When searching for materials on art theory, use art together with the following keywords: theory, aesthetics, philosophy, criticism. When researching art history as a discipline, use the subject heading: art historiography.
Grove Art Online provides web access to the entire text of The Dictionary of Art, ed. Jane Turner (1996, 34 vols.) and The Oxford Companion to Western Art, ed. Hugh Brigstocke (2001). The University of Manitoba's subscription also includes access to: The Oxford Companion to Western Art; Encyclopedia of Aesthetics: The Concise Oxford Dictionary of Art Terms.